1. Field of Invention
The invention relates to an active matrix substrate inspecting method, an active matrix substrate, a liquid crystal device and an electronic apparatus. In particular, the invention relates to techniques for inspecting a type of active matrix substrate in which a digital data-line driver, i.e., a driver in which a digital signal is input and converted from digital to analog form to output an analog signal for driving data lines, (hereinafter referred to as a xe2x80x9cdigital driverxe2x80x9d) is formed on the substrate.
2. Description of Related Art
In recent years, research has been conducted on an active matrix substrate with a built-in driver in which driving circuits (drivers) for scanning lines and data lines are formed on a substrate, and a liquid-crystal display device using the substrate. Such an active matrix substrate is produced using low-temperature polysilicon techniques, for example. In order to sell the products, which use the above active matrix substrate, on the market, it is necessary to accurately perform a testing of good or bad products from the perspective of guaranteeing reliability prior to the panel assembly after the substrate is formed.
According to a study conducted by the present inventor, the above-described inspection mainly requires basic inspections, such as checking the drivers"" output ability and detecting a disconnection of the data lines, and inspections of the characteristics of switching devices (TFTs, MIMs, etc.) included in pixels as well as the leak characteristics of storage capacitors, which relate to an inspection on a point defect in an active matrix unit.
In the case of a digital driver for driving the data lines (namely, digital-data-line driver), a method for performing simultaneous driving at predetermined timing (line-at-a-time driving method) has been employed by paying attention to the ease of digital data storing.
A display device with such built-in digital-line-at-a-time-driving driver has not been realized, so that the manner in which the above-described highly reliable inspections are performed is not clear.
Therefore, one of the objects of the invention is to establish a testing technology of the active matrix substrate in which a digital driver is loaded and to sell substrates, display devices, or the like with high reliability on the market.
In order to accomplish this object, the invention has the following structures.
The invention includes:
a plurality of scanning lines and a plurality of data lines;
a digital driver that enables an output terminal to be in a high-impedance condition, the digital driver being provided for driving the plurality of data lines;
switching devices connected to each scanning line and each data line;
capacitors connected to the each of switching devices; and
an inspection circuit provided at ends of the data lines opposite to the digital driver, the inspection circuit including bi-directional switches provided for each of the plurality of data lines, and control means for controlling the switching of the switches.
Because the digital driver for the data lines has a D/A converter in an output unit, an active matrix unit testing (measurement of point defects) cannot be performed by reading the signal again which was once output through the common channel.
However, according to the invention, since the inspection circuit is provided at the opposite ends of the data lines to the digital driver, signals can be written in the capacitors (storage capacitors) in the active matrix unit by driving the data lines with the digital driver, and the written signals can be read out through the inspection circuit. Accordingly, determination of whether or not there is a point defect can be made.
In the case of the signal reading with the inspection circuit, when the output of the digital driver (A/D conversion output) is turned on, defect determination based on signals read out from the storage capacitors is not secured. Thus, in order to acquire a basic signal for point-defect determination, the output of the inspection circuit needs to be switched off (set to be in a high-impedance condition). Therefore, the digital driver includes the function of enabling the output to be in a high-impedance condition.
In addition, since the inspection circuit is provided as a circuit used for inspection, it does need to operate at high speed like the digital driver, and it only needs a minimum function, for example, being capable of performing inspection. Accordingly, in accordance with the invention, the digital driver has a structure including bi-directional switches, e.g., analog switches, and control means for controlling the switching of the switches. The digital driver""s simplified circuit arrangement and the fact that it is not required to have advanced operating characteristics enables it to be a small transistor size, which provides the advantage of space reduction. Therefore, the inspection circuit can be mounted on the active matrix substrate easily.
The xe2x80x9cinspection circuitxe2x80x9d means a circuit that is mainly used for inspection and not having the function of driving the data lines like the digital driver. However, the xe2x80x9cinspection circuitxe2x80x9d can have another object which is different from inspection, and it can include a component that can be used to accomplish an object other than inspection.
Devices included in the inspection circuit are produced, together with devices included in the digital driver, by an identical production process.
On one active matrix substrate, a digital driver and an inspection circuit are produced by an identical process. For example, using low-temperature polysilicon-thin-film-transistor (TFT) techniques enables their production.
The digital driver includes a switch in its output unit, and opening the switch causes the output unit to be in a high-impedance condition.
The switch is provided in the output unit in order to make the output of the digital driver be in a high-impedance condition.
The digital driver includes any one of a switched capacitor D/A converter, a resistor ladder D/A converter and a PWM D/A converter.
Examples of a D/A converter that is mountable on an active matrix substrate in accordance with the invention are described below.
In a switched capacitor D/A converter, for example, switches are provided for each of the weighted capacitors, and the charge of each capacitor is combined with a coupling capacitor by control of the switching of the switches so as to generate a conversion voltage.
In a resistor ladder D/A converter, for example, a resistance-divided voltage is selectively extracted by control of the switching of the switches provided at the output channels so as to generate a conversion voltage.
In a PWM D/A converter, for example, the on-duration of a switch connected to a voltage source in which a voltage value varies with time (ramp-wave) is controlled in accordance with a digital data value so as to generate a conversion voltage.
The control means in the inspection circuit performs point-at-a-time scanning of the bi-directional switches.
The inspection circuit has, for example, a point-at-a-time scanning system of data lines using shift registers, etc. and performs inspection by the point-at-a-time reading of data.
When the number of the bi-directional switches is M, where M is a natural number not less than 2, the control means in the inspection circuit repeatedly performs the simultaneous driving of P, where P is a natural number, bi-directional switches Q, where Q is a natural number, times, whereby the driving M (M=Pxc3x97Q) of bi-directional switches in total is realized.
The inspection circuit uses a method different from point-at-a-time scanning.
At least a part of the inspection circuit is disposed in a space, which is in the active matrix substrate and not contributing to the realization of substantial functions, such as displaying an image.
Since the inspection circuit only needs to be a small transistor size and thus only occupies a small area, at least a part of the inspection circuit can be disposed in a space, which is in the active matrix substrate and not contributing to the realization of substantial functions, such as displaying an image, namely, a so-called dead space. Therefore, the enlargement of the active matrix substrate and liquid-crystal display device can be suppressed.
The inspection circuit is disposed in a sealing position formed by sealing material in a panel production process.
The position to be sealed by the sealing material in a panel production process is a dead space inevitably generated in the active matrix substrate. By disposing the inspection circuit in this space, the effective use of space can be achieved.
Each of the inspection circuit and the digital driver is divided into multiple pieces and disposed on the active matrix substrate.
There are cases when a further effective use of the dead space is possible by disposing circuits separately. Furthermore, a layout design with extra spaces is possible when the number of devices in one block decreases only for the separated part. Moreover, it is also possible to decrease the number of the operation frequency of the circuits which operate in series a shift register or the like only for the amount in which the number of devices decreased.
The inspection circuit is separated into at least a first inspection circuit and a second inspection circuit, while the digital driver is separated into at least a first digital driver and a second digital driver.
The first digital driver and the first inspection circuit are disposed so as to be opposed, with the data lines provided therebetween. The second digital driver and the second inspection circuit are disposed so as to be opposed, with the data lines provided therebetween.
The first digital driver and the second inspection circuit are disposed at identical ends of the data lines, and the second digital driver and the first inspection circuit are disposed at identical ends of the data lines.
A layout is formed in which the separated identical-type circuits, each of the first circuit and the second circuit, are disposed at the opposite sides of the data lines provided between them.
Since the circuits are dispersed and disposed on the top and bottom surfaces of the active matrix substrate, dead space around the display region is easy to effectively use. In particular, there are dead spaces, uniformly at sealing positions around, at the top and bottom of, the substrate, which is advantageous when the spaces are effectively used.
In addition, the circuit separation reduces the number of devices in one circuit block in accordance with the number of separations, which provides enough space for the layout. It is possible to decrease the number of operation frequency of the circuits which operate in series a shift register or the like only for the amount in which the number of devices decreased.
There is provided a method for inspecting an active matrix substrate, the method comprising the steps of:
writing signals in the capacitors connected to the switches by using the digital driver to drive the data lines;
causing the output of the digital driver to be in high-impedance condition; and
acquiring a basic signal to be a basis of inspection by reading the signals written in the capacitors by the inspection circuit, and inspecting an active matrix unit, based on the acquired signal.
Next, a basic method for inspecting an active matrix unit using an active matrix substrate is described as follows.
The inspection of the digital driver itself and the inspection for the data line disconnection are performed before each step is executed.
Before the active matrix unit is inspected, the output characteristics of a digital driver itself and the disconnection of data lines are inspected (preliminary inspection).
In the active matrix substrate of the invention which was described above, the inspection circuit is disposed so as to oppose a digital driver, with data lines provided between them. Thus, if each data line is line-at-a-time driven, for example, just once by a digital driver, the inspection circuit is scanned by a line or point sequence as it is synchronized with the scanning, and the signal which is transmitted through data line is received, it is possible to easily perform a preliminary inspection by the receiptor or the amplitude of the received signals or the like.
The step of inspecting the active matrix unit, based on the acquired basic signals, includes the step of considering the two-dimensional distribution of the basic signal characteristics in the active matrix unit.
In many cases, the basic signal to be acquired for inspecting the active matrix unit includes considerable noise. Accordingly, it is effective to perform relative inspection with consideration on not only the absolute value of the signal characteristic but abnormality in the signal characteristic distribution, e.g., the existence of a particular portion indicating abnormality in the form of a significant point, compared with the neighborhood, and so on.
The step of inspecting the active matrix unit, based on the acquired basic signal, includes the step of comparing the acquired basic signal with a sample signal, which is previously prepared.
This is a method of inspecting by using the comparison of the sample signals. Furthermore, the liquid crystal device is structured by using an active matrix substrate, and this is a liquid crystal with high reliability which passed a certain inspection. The electronic equipment which was structured by using this liquid crystal has a high reliability of the device so that reliability of the electronic equipment can also be improved.